mysql> check table sessions;
+-------------------+-------+----------+----------------------------------------------------------+
| Table | Op | Msg_type | Msg_text |
+-------------------+-------+----------+----------------------------------------------------------+
| torkalot.sessions | check | warning | 7 clients are using or haven't closed the table properly |
| torkalot.sessions | check | error | Found 2 keys of 1 |
| torkalot.sessions | check | error | Corrupt |
+-------------------+-------+----------+----------------------------------------------------------+
3 rows in set (54.76 sec)
mysql> repair table sessions;
+-------------------+--------+----------+----------+
| Table | Op | Msg_type | Msg_text |
+-------------------+--------+----------+----------+
| torkalot.sessions | repair | status | OK |
+-------------------+--------+----------+----------+
1 row in set (0.20 sec)
mysql> check table sessions;
+-------------------+-------+----------+----------+
| Table | Op | Msg_type | Msg_text |
+-------------------+-------+----------+----------+
| torkalot.sessions | check | status | OK |
+-------------------+-------+----------+----------+
1 row in set (0.00 sec)
mysql>
So in this case, there appeared to be 3 issues reported by the check tool - there were 7 cases of clients which had opened the table, but not closed it properly (presumably not releasing locks on disconnect probably due to an unclean shutdown). I'm guessing, but probably the 'Found 2 keys of 1' would suggest a duplicate primary key situation.
Anyway, running the 'check table' and 'repair table' commands will save your day. You could equally run myisamchk from the command line.
christo